Geometric Unsharpness vs. Penetrameter Placement on a Complex Casting
You are radiographing a 40 mm thick steel casting with a complex internal cavity using Ir-192. The part cannot be rotated easily, and the only accessible source position gives an SOD of 350 mm. The focal spot is 3.5 mm. The client requires a 2-1T IQI on the source side and a maximum Ug of 1.5 mm. The radiographer places the IQI on the film side because 'the source side is not accessible' and reports that the film 'passes' because the 2T hole is visible. The film density is 2.4. You must determine compliance and decide whether to accept, re-shoot, or revise the technique.
1. Calculate Ug and determine whether the geometry meets the 1.5 mm limit. State the effect of IQI placement on the validity of the sensitivity result.
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Ug = Ft/(SOD - t) = 3.540/(350-40) = 140/310 = 0.45 mm. This is well under 1.5 mm, so geometry is compliant.
IQI placement: the client requires source-side placement. Film-side placement is only permitted when source-side is physically impossible, and then a code-specified compensation (e.g., a larger IQI or a specific hole) applies. Here the radiographer used film-side placement and reported the 2T hole visible - but with a 2-1T IQI on the film side, the sensitivity demonstrated is not equivalent to source-side 2-1T. Film-side IQIs overstate sensitivity because the IQI is closer to the film and less affected by geometric unsharpness and scatter. The result cannot be accepted as meeting the source-side 2-1T requirement without the code's film-side compensation rule.
2. The radiographer says the source side is inaccessible. What are your options, and which do you recommend?
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Options:
- Reposition the source or part to achieve source-side IQI placement (preferred). Even a modest change in source position may make the source side accessible.
- Use a film-side IQI with the code-permitted compensation: typically a larger IQI (e.g., 1-1T instead of 2-1T) or the specified hole, and document the justification. This is acceptable only if source-side placement is truly impossible.
- Use a different technique (e.g., a different source position or a double-wall technique) that allows source-side placement.
Recommendation: first attempt to reposition to achieve source-side placement, since the geometry already has margin (Ug = 0.45 mm vs. 1.5 mm limit) and the only obstacle is access. If truly impossible, apply the code's film-side compensation and document it in the technique sheet with the specific clause. Do not accept the film-side 2-1T result as equivalent to source-side 2-1T.
3. If you must use film-side placement with compensation, what IQI size and hole would you specify, and how would you verify the result?
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Under the common code rule, film-side placement requires a larger IQI: for a 2-1T source-side requirement, film-side typically requires 1-1T (one size larger) or the code-specified equivalent. So specify a 1-1T IQI on the film side, or the exact compensation stated in the applicable code (some codes require the 1T hole to be visible with a 1-1T IQI).
Verification: confirm the 1T hole is visible under the required viewing conditions (appropriate illuminance, densitometer-verified density 2.0-4.0, no glare), record the IQI designation and placement on the technique sheet, and have the Level III sign off on the deviation with the code clause cited. Also confirm the density remains in range and that the IQI is in a region representative of the weld/casting, not in a thicker or thinner area.